1. Field of the Invention
This invention relates to a phase measurement system for a moving body having a row of magnets therein, and more particularly to phase measurement system for a magnetically controlled signal by which the phase difference of the magnetically controlled signal can be read out with high accuracy.
2. Description of the Prior Art
Multi-gap magnetic heads as shown in FIG. 1 have hitherto been used for the read-out of the length of phase difference of a magnetically controlled signal.
Referring to FIG. 1, a pair of multi-gap magnetic heads 20A and 20B has a plurality of gaps g and selectivity of wave length. The multi-gap magnetic heads 20A and 20B comprise a plurality of magnetic heads 20 (FIG. 2) connected in series with each other and at regular intervals. The magnetic head 20 has a lateral core 21 on which a coil 22 is wound.
The selectivity of wave length means the function to detect selectively signals with specific frequencies from a composite signal with different frequency components. When the multi-gap magnetic heads detect the repeated magnetic flux changes which are formed by the row of magnets, the selectivity of wave length means the function to detect selectively flux changes with wave lengths twice, four times, six times . . . . as long as the distance between the adjacent gaps of the multi-gap magnetic head.
When a normal magnetic head runs parallel to the row of magnets, the frequency and amplitude of the detected sine wave output vary with the running speed of the magnetic head. When the magnetic head stops, no output is obtained from the magnetic head.
In the prior art measurement system, an alternating voltage with the frequency ##EQU1## IS APPLIED TO THE MAGNETIC HEAD 20 SHOWN ON FIG. 2. The alternating voltage is sufficient to almost saturate the magnetic head. Output with the frequency f is always obtained from an output terminal 23. When the magnetic head 20 is stopped, output is obtained from the magnetic head 20 in response to the magnetic flux occurring at the stop position.
The multi-gap magnetic heads 20A and 20B are of a responding type magnetic flux. The distance between the multi-gap magnetic heads 20A and 20B is ##EQU2## where n = 0, 1, 2 . . . and .lambda.m a wave length, as shown on FIG. 1. An alternating voltage ##EQU3## is applied to the one multi-gap magnetic head 20A. Another alternating voltage ##EQU4## shifted from the voltage ##EQU5## by 90.degree. in phase, is applied to the other multi-gap magnetic head 20B. When the multi-gap magnetic heads 20A and 20B are displaced by x from the position shown on FIG. 1, keeping the distance between the magnetic heads 20A and 20B, the following output voltages V.sub.A and V.sub.B are obtained from the output terminals 23 and 23': ##EQU6##
h in the above equations is a constant depending on the shapes of the multi-gap magnetic heads 20A and 20B, the gap loss, the selectivity of wave length and so on. The constant is determined by the distance .lambda.e between the adjacent gaps, the distance ##EQU7## between the multi-gap magnetic heads 20A and 20B, the wave length .lambda. m between the multi-gap magnetic heads 20A and 20B, the wave length .lambda. m of the recorded signals, and so on. The terms ##EQU8## in the above equations represent the changes of the amplitudes of the output voltages V.sub.A and V.sub.B. The carrier wave V sin .omega.t is amplitude-modulated by ##EQU9## with respect to the output voltage V.sub.A. The carrier wave V cos .omega.t is amplitude-modulated by ##EQU10## with respect to the output voltage V.sub.B. The output voltage V.sub.A is added to the output voltage V.sub.B, to form a magnetically controlled signal V.sub.P. ##EQU11##
If the displacement x is an integral number of times as long as the wave length .lambda. m, it can be read out by the selectivity of wave length, of the multi-gap magnetic heads 20A and 20B. When the displacement x is not an integral number times as long as the wave length .lambda. m, or when the multi-gap magnetic heads 20A and 20B stop at the position where the gaps g deflect from the corresponding border lines of the adjacent magnets, the deflection cannot be read out by the selectivity of wave length.
A phase difference represented by a length shorter than the wave length .lambda. m can be measured by the measurement method using a circuit shown on FIG. 3.
In FIG. 3, a reference signal R is applied to a Schmidt circuit 25. A signal V sin .omega.t is used as the reference signal R which is shown on FIG. 4(a). A rectangular wave shown on FIG. 4 (c) is obtained from the Schmidt circuit 25. The wave form of the voltage V.sub.P of the equation (3) is shown on FIG. 4 (b). The voltage V.sub.P is shifted from the reference signal R by ##EQU12## in phase. It is applied to another Schmidt circuit 26. A rectangular wave shown on FIG. 4 (d) is obtained from the Schmidt circuit 26. It is shifted from the rectangular wave of FIG. 4 (c) by ##EQU13## in phase. The rectangular waves of FIG. 4 (c) and FIG. 4 (d) are applied to an AND circuit 27. Pulses with the width ##EQU14## are obtained from the AND-circuit 27 (FIG. 4 (e)). The pulses are applied to a gate circuit 28, as gate signals. While the pulse is applied to the gate circuit 28, clock pulses from a clock pulse generator 29 can pass through the gate circuit 28. Accordingly, output shown on FIG. 4 (f) is obtained from an output terminal 30 of the gate circuit 28. The distance between the adjacent clock pulses is a measurable unit of the phase difference represented as some fraction of the wavelength .lambda. m. Accordingly, the length corresponding to the phase difference ##EQU15## can be measured by the counting of the number of the clock pulses during one gate interval.
In FIG. 3, an integrator 31 functions to detect the direction of the movement of the multi-gap magnetic heads 20A and 20B. When the multi-gap magnetic heads 20A and 20B are moved in the right direction on FIG. 1, a saw-toothed wave shown on FIG. 4 (g) is obtained from an output terminal 32 of the integrator 31. When the multi-gap magnetic heads 20A and 20B are moved in the left direction on FIG. 1, a saw-toothed wave shown on FIG. 4 (h) is obtained from the output terminal 32 of the integrator 31. The saw-toothed wave of FIG. 4 (g) is of acclivity. The saw-toothed wave of FIG. 4 (h) is of declivity. Positive and negative pulses are obtained with the differentiations of the saw-toothed waves of FIG. 4 (g) and FIG. 4 (h), respectively. Thus, the direction of the movement of the multi-gap magnetic heads 20A and 20B can be detected, and thus, the direction of phase change.
In the above-mentioned measurement method using the multi-gap magnetic heads, many disadvantages are present. The coils 22 are required for the magnetic heads 20. For that reason, the arrangements of the measuring apparatus are complicated. The measuring apparatus is apt to be affected by the change of temperature and the external magnetic field. Since the frequency of the reference voltage is double the coil driving voltage, a frequency multiplying or dividing means is necessary.